1. Field of the Invention
The present invention relate to an organic photoconductor used in a field of copying machines and printers (hereinafter simply referred to as a photoconductor) and a process cartridge, an image forming apparatus and an image forming method using the organic photoconductor.
2. Description of Related Art
In earlier technology, in order to improve quality of photoelectorographic image, a technology forming a microscopic dot image in which a fine latent image is formed on an organic photoconductor using exposure light source of small spot diameter has been developed. For example, JP Tokukaihei-8-272197A and U.S. Pat. No. 5,818,489 disclose a method of forming a latent image of high definition on an organic photoconductor using a light source having spot diameter of 4000 μm or less. In order to form an accurate latent image in such spot exposure method of small spot diameter, it is important to reduce the diffusion of charge carrier generated by an exposure in a latent image formation by the image exposure to an organic photoconductor.
That is, in order to reproduce image information faithfully as an electrostatic latent image, it is necessary to keep enough contrast of electric potential between exposed and unexposed part. Thus, it is important to reduce the diffusion of carrier which takes place while the carrier is generated and reach surface charge. For example, Journal of the Imaging Society of Japan 38 (4), in page 296, discloses that the effect of diffusion in electric latent image formation can not ignore about a high density image of such like 1200 dpi, when the ratio of diffusion constant (D) to drift mobility (μ) in charge transport layer D/μ become large, and thus, thicker charge transport layer results larger degradation of a latent image.
Furthermore, as described in Fuji Jihou 75 (3), page 194, the larger drift mobility (μ) in charge transport layer results larger diffusion of a latent image according to an analysis result of single dot latent image. As disclosed in JP Tokukaihei-5-119503A, in a process with high resolution, an organic photoconductor has already proposed, in which thinner charge transport layer is applied and diffusion of electrostatic latent image is prevented.
However, the above-proposed organic photoconductors can not be an enough closure of the problem with respect to durability of photoconductor. That is, tear and wear of the film involved in a repeat use easily cause increase of image defects such as fog, small black spot and the like, because potential stability, sensitivity and the like of organic photoconductor largely depend on the film thickness. In particular, an organic photoconductor having thin photoconductive layer has a tendency that field intensity per film thickness included in conditions of charging potential in electrostatic latent image formation is large, and problems such as degradation of a dot image and elevation of residual potential easily occur.
Photoelectrical apparatuses such as a digital coping machine, a printer and the like in recent years are downsized speeded up as well as high image quality is required. As a result, both higher sensitivity corresponding to speeding up and longer lifetime according to improvement of abrasion resistance are required.
In order to fulfill the above requirement of high image quality, downsizing and speeding up, it is required to enhance time response property in sensitivity of organic photoconductor. In order to fulfill the requirement, a development of electrical charge generation material has been made. As a result, as described in Journal of the Electrophotography Society of Japan 29 (3), page 250 (1990), phthalocyanine pigment such as Y type phthalocyanine (titanil phthalocyanine pigments having a maximum peak at 27.2° in Bragg angle 2θ in character X ray spectrum of Cu—Kα) is developed, and a photoelectrical photoconductor using the pigment thereof is in practical use.
However, charging potential of these photoelectrical photoconductor is not stable in high-speed image forming process in which a line speed of photoconductor is fast and charged time and moving time from exposure process to developing process are short, which cause degradation of a dot image and elevation of residual potential. As a result, fog or degradation of image density easily turns up.
Consequently, in a organic photoconductor required high image quality and high speed property, it has been problematic that change of film thickness of photoconductor involved in repeat use affect size of a electrostatic latent image of dot image and formation of potential contrast, both of which cause degradation of a dot image and elevation of residual potential. As a result, fog or degradation of image density easily turns up. In particular, degradation of a dot image caused by wear and tear of photoconductor easily turn up in a print image of photographic image and the like where a dot image of 1200 dpi or more (dpi is referred to the number of dot in 2.54 cm) is required and reproducibility of gradation is valued. It is necessary to prevent that.